1. Field of the Invention
The present invention generally relates to a method and apparatus for preserving the integrity of thin refractory metal films when the films are subjected to a predetermined high temperature in an oxidizing atmosphere.
More particularly, the present invention relates to preventing oxidation of the metal film and to preventing the metal film from delaminating from the substrate onto which it has been deposited.
2. Description of the Related Art
With their high conductivity, their high melting points, and the values exhibited by their work functions, refractory metals are attractive materials for incorporation into semiconductor devices. For example, tungsten is potentially an attractive material from which to fabricate the gate electrodes of field effect transistors (FETs). To form such structures, the refractory metal films can be deposited onto the semiconductor substrate, typically comprising a silicon dioxide layer on a silicon substrate, by any of a plurality of methods. The refractory metal can also be incorporated into the structure by, for example, wafer-bonding techniques.
However, a problem arises when the refractory metal, already incorporated into the device structure, must be subjected to an oxidizing atmosphere at a relatively high temperature (e.g., greater than about 300.degree. C. to about 700.degree. C. depending upon the structure being built and the specific processes involved). For example, as shown in FIGS. 1A-1B, steps in the fabrication of a structure are illustrated in which the refractory metal (e.g., tungsten) is to be employed as the back gate of a double-gated FET device. By a combination of steps, involving W deposition and subsequent wafer bonding, a structure such as that illustrated schematically in FIG. 1A can be fabricated.
In FIG. 1A, first a silicon wafer substrate formed of an oxide layer 2 on a silicon substrate is provided. Thus, the wafer substrate is a thick insulating oxide layer. Further, a tungsten layer 3 is provided on the oxide layer 2. The tungsten layer 3 forms the back gate electrode in the finished structure. A back gate oxide 4 having a thickness of approximately 5.0 nm, is applied over the tungsten layer. Additionally, silicon layer 5 is provided on the oxide layer 4. The silicon layer 5 forms the channel of the FET in the finished device.
Thereafter, the next step is to form the top gate oxide 6, as shown (not to scale) in FIG. 1B. To grow this oxide 6, a preferred procedure is to place the wafer in a reactor at a temperature of approximately 700.degree. C. in 1 atmosphere of pure oxygen for a period of about 5-10 minutes or longer.
However, when the processing steps depicted in FIG. 1A are attempted and a high temperature processing in an oxidizing atmosphere are performed, the tungsten layer 3 may become delaminated from the insulating oxide 2 (e.g., at interface A shown in FIG. 1B), thereby destroying the structure and its utility. Even at lower temperatures (e.g., about 300.degree. C.), in attempting to perform oxidation in pure oxygen or depositing oxides from silane-oxygen mixtures, deleterious effects have been observed.
To validate that this behavior is the result of the inherent instability of interface A at high temperature in oxidizing ambients, and not to any of the details in the processing necessary to form the structure in FIG. 1A, experiments were performed on the simplified structure 20 shown in FIG. 2A.
The structure of FIG. 2A is simply a blanket film of tungsten 23 deposited on SiO.sub.2 22 thermally grown on a silicon substrate 21. The tungsten 23 was deposited by chemical vapor deposition (CVD) from W(CO) 6 to a thickness within a range of about 100 .ANG. to about 1000 .ANG.. Such films may be heated to about 1000.degree. C. in an inert atmosphere (e.g., Argon or moderate vacuums with pressure less than about 10.sup.-4 torr) in a rapid thermal annealing system without apparent degradation. However, heating these structures in an oxidizing ambient (1 atmosphere of pure oxygen) caused the metal film to visibly oxide and/or delaminate at temperatures as low as about 300.degree. C.
Moreover, no film was ever observed to withstand the time/temperature combination required to grow the top gate oxide 6 of FIG. 1B. Therefore, without a new method to stabilize interface A, the structure of FIG. 1B cannot be fabricated. That is, the refractory metal film will delaminate at the interface, or at the very least the refractory metal (e.g., tungsten or the like) may still hold, but be oxidized, thereby causing stability or performance problems of the device.